The present invention relates to a method for producing a biaxially oriented oxygen and moisture barrier film which is comprised of a polyolefin layer and an ethylene vinyl alcohol copolymer barrier layer. More particularly, the invention relates to a simple continuous method whereby the two layers are combined by co-extrusion and biaxially oriented at the same time.
Ethylene vinyl alcohol copolymer resins offer excellent barrier properties with respect to such gases as oxygen, carbon dioxide and nitrogen. In addition, they are also effective barriers against odors and the loss of flavor. Such resins, hereinafter referred to as EVOH resins, are moisture sensitive and the barrier properties are reduced in the presence of high humidity. Polypropylene offers excellent barrier properties with respect to moisture together with good strength properties and a high use temperature. When EVOH resins are encapsulated by layers of polypropylene, they are protected from moisture and therefore retain their barrier characteristics.
The biaxial orientation of EVOH resins enhances their barrier properties as well as reduces their susceptibility to moisture. The biaxial orientation of polypropylene increases its stiffness and enhances both its optical and other physical properties such as tensile strength, tear strength, and other mechanical properties.
In the past, others have attempted to produce biaxially oriented composite structures containing both polypropylene and EVOH resins by first producing a polypropylene sheet and orienting the sheet in the longitudinal direction. Then a layer of EVOH resin was either laminated or extrusion coated onto the polypropylene and the composite was then oriented in the transverse direction. This method of production is costly and only results in the orientation of the EVOH resin in one direction, thus not achieving the full benefit of biaxial orientation.
U.S. Pat. No. 4,239,826, issued Dec. 16, 1980, discloses a multi-layer barrier film comprising a barrier layer of substantially pure ethylene vinyl alcohol copolymer adhered between adjacent adherent layers consisting essentially of a partially hydrolyzed vinyl acetate polymer or copolymer. A layer of another material such as polypropylene can overlie the partially hydrolyzed vinyl acetate layers. The patent discloses co-extrusion of the EVOH and the partially hydrolyzed vinyl acetate polymer to form the multi-layer structure and then subsequent co-extrusion of the overlying material onto this structure. The patent does not suggest that this construction could be biaxially oriented and is otherwise distinguishable from the present invention because the adhesive is different, it does not mention controlling crystallinity of the EVOH, the percent ethylene of the products mentioned is too low for flexibility for biaxially oriented film, and, even if this film was to be biaxially oriented, the operation would be a two-step operation whereas the operation of the present invention is a one-step operation.